The requirements for a joint protecting a connection between a power cable (such as those used in transmission and distribution networks) and another cable or other electrical equipment, such as switch gear or a transformer, are more demanding than those for joint protecting a signal (e.g., telephone) cable connection. This is especially true with high voltage power cables. (Generally, "high voltage" denotes voltages above 1 kV, with the subrange of 1-69 kV being referred to herein as "medium voltage.") In both instances, a good seal which protects the conductors from moisture, dust, and other environmental contaminants is essential. However, in a signal cable joint, the dielectric strength of the insulation is not critical, provided it is a good insulator. With a high voltage cable joint, the insulation must resist dielectric breakdown at the much higher voltage stresses encountered. Also, power equipment operates at higher temperatures than telephone or other signal cables, so high temperature stability is needed. Finally, electrical stress must be controlled by appropriate design of the closure or by placement of stress grading material at critical locations to distribute the stress. (Herein, references to "conductive," "conductivity," and "stress" are to be construed in the electrical context, unless indicated otherwise.)
A common design for high and low voltage joints has a tubular outer protective component. The component is slid over one of the cable ends, temporarily "parked" next to the connection area, and then slid over the connection area after completion of the conductor splicing operation. This approach necessitates extra working area at least equal to the length of the joint itself. As the splicing may take place in cramped quarters, the additional space may be unavailable. For high voltage power cables, with typically longer joints, this problem is accentuated.
An alternative design employed with signal cables closures has two half shells, each pre-filled with a void-filling material. The two half shells are fitted together over the connection, with the void-filling material encapsulating the connection and cables. This design is attractive because it does not have any components which must be slid off to a side and environmental sealing is achieved by simply mating the two halves together. Many examples exist in the low-voltage area, for example Dobbin et al., U.S. Pat. No. 3,879,575 (1975); Reuter, U.S. Pat. No. 4,849,580 (1989); Jervis, U.S. Pat. No. 4,859,809 (1989); Jervis, U.S. Pat. No. 4,909,756 (1990); Usami et al., U.S. Pat. No. 5,099,088 (1992); and Jervis, U.S. Pat. No. 5,173,173 (1992). However, this design has not been used in the high voltage area because the interface between the void-filling material in the two shells can provide a path through which dielectric failure can readily occur. It has been reported that the interfacial dielectric strength between two pieces of the same material is about one-sixth the bulk dielectric strength of material itself. Fournier et al., "Interfacial breakdown phenomena between two EPDM surfaces," Pub. No. 363, 6th IEE Conference on Dielectric Materials, Measurements, and Applications, pp. 330-333 (Sep. 1992, Manchester, U. Kingdom). It has also been reported that dielectric breakdown by tracking at interfaces accounts for about 40% of the failures in medium voltage cable splices. Lamarre et al., "Characterization of medium voltage cable splices aged in service," Proceedings of the Jicable 91 International Conference, Versailles, France, pp. 298-304 (Jun. 1991). Further, low voltage pre-filled half-shell closures are unsuitable for high voltage applications because they make no provision for controlling stress.
Where the half-shell design has been employed in the prior art for high voltage joints, pre-filling has been avoided: untilled half-shells are positioned around a connection and an encapsulant (e.g., a polyurethane or bitumen) is poured in through a vent and allowed to set. In this "cast-resin" approach the encapsulant sets as a single mass, avoiding the creation of an interface within the encapsulant along which failure can occur. See, for example, Reynaert, U.S. Pat. No. 4,943,685 (1990). However, the joint cannot be inspected, tested, or buried, nor the cable energized, until after the encapsulant has set--which may be an extended period of time. Also, cast resin closures are normally not re-enterable.
Thus, it is desirable to provide a closure for high voltage connections overcoming the foregoing limitations, which employs a pre-cured void filling material which is not susceptible to interfacial dielectric failure.